1. Field of the Invention
This invention relates to a wafer cassette and a liquid-phase growth system and a liquid-phase growth process which make use of the wafer cassette, and is particularly applicable to a liquid-phase growth process and a liquid-phase growth system which are of an immersion type in which wafer-size substrates are held with a jig and immersed in a solvent.
2. Related Background Art
Combustion of oil for thermal power generation, combustion of gasoline by automobile engines and other combustion have come to be the cause of pollution of the global environment. There is also an anxiety about the exhaustion of crude oil. Accordingly, an increasing interest is taken in solar-cell power generation as a clean energy source.
Thin-film crystal-silicon (Si) solar cells have so thin power-generating layers as to allow the use of silicon materials in a small quantity, and enable cost reduction. Also, since crystal silicon is used in the power-generating layers, a higher conversion efficiency and a lower deterioration can be expected, compared with amorphous-silicon solar cells. Moreover, such thin-film crystal-silicon solar cells can be bent to a certain extent, and hence can be attached to curved surfaces of automobile bodies, household electric appliances, roof tiles and so forth when used.
To materialize the thin-film crystal-silicon solar cell, Japanese Patent Application Laid-Open No. 8-213645 discloses that a thin-film single-crystal silicon is separated utilizing an epitaxial layer grown on a porous silicon layer by CVD (chemical vapor deposition). FIG. 18 is a cross-sectional view showing a method of forming a thin-film silicon solar cell in this Japanese Patent Application Laid-Open No. 8-213645. In FIG. 18, reference numeral 101 denotes a silicon wafer; 102, a porous silicon layer; 103, a p+-type silicon layer; 104, a p−-type silicon layer; 105, an n+-type silicon layer; 106, a protective layer; 109 and 111, adhesives; and 110 and 112, jigs. In the method of producing a solar cell as shown in FIG. 18, the porous silicon layer 102 is formed on the surface of the silicon wafer 101 by anodizing. Thereafter, the p+-type silicon layer is epitaxially grown on the porous silicon layer 102, and the p−-type silicon layer 104 and n+-type silicon layer 105 are further epitaxially grown thereon. Then, the protective layer 106 is formed. Subsequently, the adhesives 109 and 111 are applied to the protective layer 106 and the silicon wafer 101 to make them adhere to the jigs 110 and 112, respectively. Thereafter, a tensile force P is caused to act on the jigs 110 and 112 to separate the silicon wafer 101 from the part of the epitaxial silicon layers 103, 104 and 105 at the part of the porous silicon layer 102. Then, a solar cell is formed in the epitaxial silicon layers 103, 104 and 105, and the silicon wafer 101 is again put into the like step so that a cost reduction can be achieved.
Japanese Patent Application Laid-Open No. 5-283722 also discloses that an epitaxial silicon layer is grown on a porous silicon layer by a liquid-phase growth process. Sn (tin) is used as a solvent, where silicon is previously dissolved in the Sn before growth and is kept saturated therein. Next, annealing is started, and, at the time the Sn solution has become super-saturated to a certain extent, the porous surface of a wafer is immersed therein to allow an epitaxial silicon layer to grow on the porous surface.
Japanese Patent Application Laid-Open No. 10-189924 still also discloses that a liquid-phase growth epitaxial layer is grown on a wafer whose surface is formed of a porous silicon layer, and the epitaxial layer is peeled to produce a solar cell.
Japanese Patent Application Laid-Open No. 10-53488 further discloses a solvent injection type liquid-phase growth system in which a jig has such a shape that any deposited films are not formed on the back and side (lateral surface). Since, however, the liquid-phase growth system disclosed in this Japanese Patent Application Laid-Open No. 10-53488 is of a solvent injection type, there is a disadvantage that the growth system must be set up on a large scale when the growth is performed on a large number of wafers. Also, in an attempt to deal with large-diameter wafers of 8 inches or larger, the growth system must be set up on a still larger scale, making this disadvantage more remarkable.
Japanese Patent Application Laid-Open No. 5-17284 still further discloses a compound semiconductor immersion type liquid-phase growth system and a holding jig. FIG. 19 is a cross-sectional view of this liquid-phase growth system. In FIG. 19, reference numeral 81 denotes a wafer holder; 82, a wafer; 83, a crucible; 84, a solvent; 85, a quartz reaction tube; 86, a gas feed pipe; 87, a gas discharge pipe; 88, a heater; and 89, a dummy wafer. In the liquid-phase growth system of this type, the wafer holder 81 having held the wafer 82 and the dummy wafer 89 is moved down (in the direction of “A”) to immerse the wafer 82 into a solvent 84 in which a growth material has been dissolved. The solvent 84 is put in the crucible 83, and the crucible 83 is placed in the quartz reaction tube 85 that keeps the atmosphere of atmospheric gas (reducing gas or inert gas) by means of the gas feed pipe 86 and the gas discharge pipe 87. The heater 88 is provided for the temperature control of the system, where the temperature of the heater 88 is dropped to lower the temperature of the solvent 84 to allow the growth material to deposit from the solvent 84 onto the wafer to effect liquid-phase growth. Compared with slide boat type or solvent injection type liquid-phase growth systems, the immersion type liquid-phase growth system enables the growth system to have a small size as long as the liquid-phase growth is performed on wafers having the same size. Also, since a large number of wafers can be arranged on the holder, the system is convenient for its adaptation to mass production.
In the case where a deposited film should be grown only on the wafer surface as in the above Japanese Patent Application Laid-Open No. 8-213645, it is desired that any deposited film is not grown on the back and side of the wafer and on the peripheral region of its surface. However, where the wafer is immersed in the solvent to perform the growth using the liquid-phase growth system disclosed in Japanese Patent Application Laid-Open No. 5-17284 like that shown in FIG. 19, the deposited film is necessarily grown not only on the whole surface of the wafer but also its back and side. Hence, where a deposited film on only the desired surface should be used, the deposited film on the back and side and on the peripheral region of the surface must be scraped off or etched away. This not only makes the number of steps larger, but also results in a low yield.
In the case of the wafer holder disclosed in Japanese Patent Application Laid-Open No. 10-53488, the liquid-phase growth can be prevented at some part of the back and side, but there is a problem that the growth takes place inevitably at some part of the side.
In the case where the liquid-phase grown epitaxial layer should be separated from the wafer as a thin-film single-crystal layer like that in the above Japanese Patent Application Laid-Open No. 10-189924, the layer may be too thick especially at the peripheral region of the surface, or, even if not thick, the layer is not successfully separated in some cases at the peripheral region because of unstableness of the porous silicon layer at the peripheral region.